A major problem with static random access memory (SRAM) scaling is the role of doping fluctuations. It is expected that for the 45 nm node doping fluctuations will be one of the single most important inhibitors for further scaling.
Projecting the threshold mismatch from the current available data, a one sigma mismatch of 80 mV is expected. This would result in an array of fail rates that is beyond a practical repair rate.
Doping fluctuation scaling is related to the absolute number, N, of doping atoms in the channel and their distribution, where N is proportional to the device area. The doping fluctuation will scale approximately to the equation 1/sqrt(W*L), where W and L are the device width and device length, respectively. Furthermore, the threshold variation is scaled by the square root of the second moment of the doping distribution perpendicular to the gate interface, which is related to the total number of dopants N in the channel. Shrinking feature sizes will require higher doping levels and because SRAM devices have one of smallest device areas known in the semiconductor art, doping fluctuation will tend to impair this region first.